This invention relates to a composite phosphor screen for detecting radiation, particularly X-rays, utilizing nanocrystalline sized phosphors (nanophosphors) disposed in extremely small channels (nanochannels) etched in a substrate.
Fine detail visualization, high-resolution high-contrast images are required for many X-ray medical imaging systems and particularly in mammography. The resolution of x-ray film/screen and digital mammography systems is currently limited to 20 line pairs/mm and 10 line pairs/mm, respectively. One of the key reasons for this limitation is associated with the phosphor particle size in the currently used x-ray screens. In particular, light scattering by the phosphor particles and their grain boundaries results in loss of spatial resolution and contrast in the image. In order to increase the resolution and contrast, scattering of the visible light must be decreased. Scattering can be decreased by reducing the phosphor particle size while maintaining the phosphor luminescence efficiency. Furthermore, the x-ray to light conversion efficiency, the quantum detection efficiency (e.g. the fraction of absorbed x-rays converted to visible light) and the screen efficiency (e.g. The fraction of emitted light escaping from the screen to expose the film) must not be negatively affected by the reduction of the phosphor particle size. The present invention is directed to a novel nanophosphor nanochannel composite screen design, which provides high resolution, high contrast, and efficient X-ray to visible light conversion screens for X-ray imaging. The composite phosphor screen can be used in both electronic (digital) and film (analog) x-ray imaging.
This work is based on the discovery of efficient doped nanocrystalline (DNC) phosphors in the 2-5 nm range. In U.S. Pat. No. 5,637,258 issued Jun. 10, 1997; of which I am a co-inventor; there is disclosed a method for producing rare earth activated (Doped) metal oxide nanocrystals, ie. phosphors These phosphors show very high conversion efficiencies from UV to visible radiation to visible at ultrafast speeds. Measurements show that the conversion efficiency of Y.sub.2 O.sub.3 :Tb DNCs is comparable to that of standard phosphors when excited at a UV wavelength of 254 nm. In my U.S. Pat. No. 5,446,286 issued Aug. 29, 1995 various radiation detectors using DNC's are disclosed. The present invention provides a screen grid arrangement that provides improved performance and reduced light scattering over the detectors of U.S. Pat. No. 5,446,286. The disclosures of U.S. Pat. Nos. 5,637,258 and 5,446,286 are hereby incorporated by reference as if fully set forth herein.
A conventional x-ray screen, as shown in FIG. 1 herein has a thickness of about 30-100 microns (.mu.m) and consists of phosphor particles with a mean size between a few to 10 microns. The light generated in the screen by the incident x-rays diffuses towards the film emulsion over the finite thickness of the screen material. As the light diffuses it spreads out which results in a loss of spatial resolution and contrast in the image. To improve resolution and contrast, thinner screens would need to be employed. However, use of the standard larger-particle phosphors in thin screens, result in grainy images and poor resolution. It is therefore necessary to significantly reduce the phosphor particle size. Thinner screens with significantly smaller phosphor particles allow for very dense packing. Thus, x-ray absorption is not reduced.
One of the major challenges in reducing the particle size below 1 .mu.m, lies in the precipitous decrease of the phosphor luminescence efficiency. This is attributed to the surface-related nonradiative processes that become dominant in the region between 1 to 0.01 .mu.m and depicted schematically in FIG. 2. This reduction in luminescence efficiency has prevented the use of smaller particles in the past. However, the introduction of the activator (dopant/light emitting atom) in particles of sizes less than 5 nm will significantly improve the light output.
Research conducted during my work on the production of doped nanocrystals demonstrated that the light generation efficiency in doped nanocrystals can be equal to or better than the best commercial phosphor. For example, the efficiency of a 3 nm size ZnS:Mn nanocrystal is reported to be 18% while the best efficiency of the bulk ZnS:Mn is about 16%. This "size-dependent" enhancement is illustrated in FIG. 2, where the phosphor luminescence efficiency increases significantly when the size of the particle is reduced below 5 nm. This research has led to the manufacture of high efficiency phosphors with particle sizes in the range of .about.3-5 nm. U.S. Pat. No. 5,637,258 is directed to a process which can produce green-emitting Y.sub.2 O.sub.3 :Tb nanocrystal phosphors which involve incorporation of rare-earth impurities in metal oxide phosphors. Chemical reactions which are carried out at less than 100.degree. C., yield particle sizes in the 3-5 nm range. The luminescence of Y.sub.2 O.sub.3 :Tb DNC phosphors has a strong dependence on particle size. This quadratic dependence is due to the quantum confinement in the nanocrystal. The nanocrystals emit light dependent upon the dopant (activator) used, thus the use of different dopants can be used to generate light of different colors.
In my U.S. Pat. No. 5,446,286 I proposed the use of films of doped nanocrystals as X-ray detection screens. Such screens are illustrated in FIG. 3 herein. The use of doped nanocrystals as the X-ray detector provides screens which are significantly faster then those of large (bulk) sized phosphors. As a comparison of FIG. 1 with FIG. 3 herein makes clear the use of nanocrystalline sized phosphors reduces the light scattering that occurs, however it does not eliminate it entirely. The present invention provides an improvement in collection efficiency over the X-ray screens of U.S. Pat. No. 5,446,286. In accordance with the present invention, the doped nanocrystals are disposed in nanochannels disposed in a substrate rather than being simply arranged in a layer on the surface of the substrate.
When glass, silicon, or metal grids with channel openings of about 0.05-5 .mu.m and aspect ratios of 200:1 to 1000:1 are packed with doped nanoparticles, a new class of high resolution composite phosphor screens become available for various medical imaging applications. By proper selection of the nanophosphor and grid materials, the X-ray generated light propagates in a waveguide mode by means of internal reflection thereby significantly reducing scattering. Thus, the nano-phosphor composite screen can dramatically enhance contrast and resolution and ensure more accurate detection and better diagnostic imaging capabilities.
The present invention permits the replacement of image intensifiers and TV camera or x-ray film in many X-ray systems. Image intensifiers are large and heavy and the combination of intensifier and camera is cumbersome in a diagnostic environment. Film images require laboratory development, are not available instantaneously and must be digitized separately to be distributed electronically. The system of the present development greatly improves the portability of the x-ray imaging systems and offers the opportunity for the real time diagnosis.
A goal of the present invention is to provide a high resolution high contrast x-ray screen which can be used both for analog (screen) and digital systems. The concept of nanochannel/nanophosphor plate provides for the first time where the x-ray radiation can be measured in both digital and analog mode. Several limitations in the existing systems such as loading factors, sensitivity, contrast and resolution are optimized and improved significantly in the proposed system. A portable x-ray imaging system capable of digital, large area imaging for teleradiology can be built. Such an X-ray imaging system would include an X-ray generator, nanochannel-nanophosphor composite screen, a built-in detector or a CCD camera and a high resolution TV monitor.
The present invention is directed to a composite phosphor screen for converting invisible radiation, such as X-rays, into visible light. The composite phosphor screen includes a planar surface, which can be formed from glass, silicon or metal, which has etched therein a multiplicity of closely spaced nanochannels having diameters on the order of 5 microns or less. Deposited within each of the nanochannels is a multiplicity of nanocrystalline phosphors, having diameters of less than 100 nanometers and preferably less than 10 nanometers, which emit light when acted upon by radiation. The walls of the nanochannels are arranged to reflect the light emitted by the nanophoshors down the nanochannels to suitable light collecting device such as film or an electronic device.
X-ray composite phosphor screens constructed in accordance with the present invention provide a number of key advantages over the prior art:
1. An X-ray system based on nanophosphors and nanochannels eliminates photon scattering, which results in higher resolution and contrast.
2. The absorption of X-rays in the phosphors can be enhanced by increasing the depth of the channel without increasing photon scattering. This means that phosphors which have relatively low X-ray absorption can be utilized in the present invention as the composite phosphor screen and can be made thicker to absorb the X-rays without loss of resolution due to increased light scattering.
3. The loading factor can be enhanced significantly by utilizing the properties of nanocrystals of materials which have high X-ray absorption such as lead oxide (PbO) and gadolinium oxide Gd2O3. Which means that the composite phosphor screen of the present invention can be utilized with systems using high energy X-rays which would otherwise damage conventional phosphor screens.
4. The doped nanocrystalline phosphors are fast decaying phosphors which allow scanning of the x-rays without the loss of resolution. Additional time integration can be used to reduce noise. Real time imaging, such as for cardiac imaging, can become feasible with these fast phosphors.
5. An integration of the present nanocrystal/nanochannel screen with Si-detector technology can yield a flat-slim x-ray imager which has a x-ray sensor on one side and a flat display on the other side.
6. The present composite phosphor screen can be optimized to be used as a Gamma-ray detector as it can be constructed so that the screen is very thick without undue light scattering, as the nanochannels form light pipes for the photons emitted by the nanocrystalline phosphors.
7. The materials used in the nanochannels and the host material of the nanophosphors can be arranged so that the light generated by the nanophosphors is collimated similar to light in an optical fiber. This effect occurs when the refractive index of the nanophosphor host material is greater than that of the material of the nanochannel or the material used to coat the inside walls of the nanochannels. This will occur if for example, if the phosphor host is Y2O3 which has a refractive index of 1.9 and the inside walls of the nanochannels are coated with SiO2, which has a refractive index of 1.5. This leads to enhancement of the light as well as elimination of cross talk between the nanochannels.
8. X-ray systems based on the present invention can be light weight and rugged thus being readily portable.